Because that requires investment. If they make that investment and are wildly successful at taking market share from x86 then ARM will become a much more valuable company. Buying ARM allows Nvidia to capture that value.

This is a real problem when some of that investment is porting/tuning various open source projects to make them run well on ARM. Such work would benefit the whole ARM ecosystem, including competitors who sell ARM based chips. At which point, owning ARM starts to look sensible.